1. Field of the Invention
The present invention relates generally to siloxane-based resins, and more specifically to the synthesis of novel siloxane based resins and the low dielectric constant films formed therefrom.
2. Related Art
Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements thus forming an integrated circuit (IC). These interconnect levels are typically separated by an insulating or dielectric film. Previously, a silicon oxide film formed using chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD) techniques was the most commonly used material for such dielectric films. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide films is problematic.
In order to provide a lower dielectric constant than that of silicon oxide, dielectric films formed from siloxane based resins are becoming widely used. One such family of films formed from siloxane based resins are the films derived from hydrogen silsesquioxane (HSQ) resins (See, U.S. Pat. No. 3,615,272, Oct. 19, 1971, Collins et al.; and U.S. Pat. No. 4,756,977, Jul. 12, 1988, Haluska et al.) However, while such films do provide lower dielectric constants than CVD or PECVD silicon oxide films and also provide other benefits such as enhanced gap filling and surface planarization, it has been found that typically the dielectric constants of such films are limited to approximately 3.0 or greater (See, U.S. Pat. No. 5,523,163, Jun. 4, 1996, Ballance et al.).
As known, the dielectric constant of such insulating films is an important factor where IC""s with low power consumption, cross-talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials, and methods for making such materials, that can provide insulating films with dielectric constants below 3.0 are very desirable. In addition, it would be desirable to have siloxane-based resins, and method for making the resins, that provide such low dielectric constant films and which additionally have a high resistance to cracking. It would also be desirable for such films to have low stress when formed in thicknesses of approximately 1.0 micron (xcexcm) or greater. Additionally, it would be desirable for such siloxane-based resins, and methods for making, to provide low dielectric constant films via standard processing techniques. In this manner curing processes that require an ammonia or ammonia derivative type of atmosphere (See, U.S. Pat. No. 5,145,723, Sep. 8, 1992, Ballance et al.), an ozone atmosphere (See, U.S. Pat. No. 5,336,532, Haluska et al.), or other non-standard type of semiconductor process, are avoided.
In accordance with the present invention, organohydridosiloxane resins, and methods for making such resins, are provided. Solutions of such organohydridosiloxane resins are employed for forming caged siloxane polymer films useful in the fabrication of a variety of microelectronic devices, particularly semiconductor integrated circuits.
The organohydridosiloxane resins of the present invention have one of the four general formulae:
(HSiO1.5)n(RSiO1.5)mxe2x80x83xe2x80x83Formula 1
(H0.4xe2x88x921.0SiO1.5xe2x88x921.8)n(R0.4xe2x88x921.0SiO1.5xe2x88x921.8)mxe2x80x83xe2x80x83Formula 2
(H0xe2x88x921.0SiO1.5xe2x88x922.0)n(RSiO1.5)mxe2x80x83xe2x80x83Formula 3
wherein:
the sum of n and m is from about 8 to about 5000 and m is selected such that the organic substituent is present to about 40 Mole percent (Mol %) or greater;
(HSiO1.5)x(RSiO1.5)y(SiO2)zxe2x80x83xe2x80x83Formula 4
wherein:
the sum of x, y and z is from about 8 to about 5000 and y is selected such that the organic substituent is present to about 40 mole percent (Mol %) or greater; and
R is selected from substituted and unsubstitued groups including normal and branched alkyl groups, cycloalkyl groups, aryl groups, and mixtures thereof;
wherein the specific Mol % of organic or carbon containing substituents is a function of the ratio of the amounts of starting materials.
Polymers in accordance with the present invention have a caged structure with a polymer backbone encompassing alternate silicon and oxygen atoms. In particular, each backbone silicon atom is bonded to at least three backbone oxygen atoms. In contrast with previously known organosiloxane resins, polymers of the present invention have essentially no hydroxyl or alkoxy groups bonded to backbone silicon atoms. Rather, each silicon atom, in addition to the aforementioned backbone oxygen atoms, is bonded only to hydrogen atoms and/or the xe2x80x98Rxe2x80x99 groups defined in Formulae 1, 2, 3 and 4. By attaching only hydrogen and/or xe2x80x98Rxe2x80x99 groups directly to backbone silicon atoms in the polymer, the shelf life of organohydridosiloxane resin solutions in accordance with the present invention is enhanced as compared to solutions of previously known organosiloxane resins.
In accordance with the methods of this invention, the synthesis of the organohydridosiloxane compositions of this invention include a dual phase solvent system using a catalyst. In some embodiments of the present invention, the starting materials encompass trichlorosilane and one or more organotrichlorosilanes, for example either an alkyl or an aryl substituted trichlorosilane.
In some embodiments, the methods of this invention include mixing a solution of at least one organotrihalosilane and hydridotrihalosilane to form a mixture; combining the mixture with a dual phase solvent which includes both a non-polar solvent and a polar solvent; adding a catalyst to the dual phase solvent and trihalosilane mixture, thus providing a dual phase reaction mixture; reacting the dual phase reaction mixture to produce an organohydridosiloxane; and recovering the organohydridosiloxane from the non-polar portion of the dual phase solvent system.
In some embodiments, additional steps may include washing the recovered organohydridosiloxane to remove any low molecular weight species, and fractionating the organohydridosiloxane product to thereby classify the product according to molecular weight.
In some embodiments, the catalyst is a phase transfer catalyst including, but not limited to, tetrabutylammonium chloride and benzyltrimethylammonium chloride. In other embodiments the catalyst is a solid phase catalyst, such as Amberjet 4200 or Amberlite I-6766 ion exchange resin (Rohm and Haas Company, Philadelphia, Pa.).
In some embodiments of the present invention, the amount of organotrihalosilane monomer present is an amount sufficient to provide an as-cured dielectric film having an organic content of at least approximately 40 Mol % carbon containing substituents. Such dielectric films formed in accordance with the present invention advantageously provide low dielectric constants, typically less than 2.7. Additionally, dielectric films in accordance with the organohydridosiloxane compositions of this invention exhibit thermal stability permitting cure temperatures of about 425 degrees Centigrade (xc2x0 C.) or greater.
As the present invention is described with reference to various embodiments thereof, it will be understood that these embodiments are presented as examples and not limitations of this invention. Thus, various modifications or adaptations of the specific materials and methods may become apparent to those skilled in the art. All such modifications, adaptations or variations that rely upon the teachings of the present invention as illustrated by the embodiments herein, are considered to be within the spirit and scope of the present invention. For example, while the embodiments herein typically use a chlorinated silane monomer, other monomers such as trifluorosilane, tribromosilane, organotrifluorosilane, and organotribromosilane can also be employed.
The organohydridosiloxane resins of the present invention have one of the four general formulae:
(HSiO1.5)n(RSiO1.5)mxe2x80x83xe2x80x83Formula 1
(H0.4xe2x88x921.0SiO1.5xe2x88x921.8)n(R0.4xe2x88x921.0SiO1.5xe2x88x921.8)mxe2x80x83xe2x80x83Formula 2
(H0xe2x88x921.0SiO1.5xe2x88x922.0)n(RSiO1.5)mxe2x80x83xe2x80x83Formula 3
wherein:
the sum of n and m is from about 8 to about 5000 and m is selected such that the organic substituent is present to about 40 Mole percent (Mol %) or greater;
(HSiO1.5)x(RSiO1.5)y(SiO2)zxe2x80x83xe2x80x83Formula 4
wherein:
the sum of x, y and z is from about 8 to about 5000 and y is selected such that the organic substituent is present to about 40 mole percent (Mol %) or greater; and
R is selected from substituted and unsubstituted groups including normal and branched alkyl groups, cycloalkyl groups, aryl groups, and mixtures thereof;
wherein the specific Mol % of organic or carbon containing substituents is a function of the ratio of the amounts of starting materials.
In some embodiments of the present invention, the substituted and unsubstituted normal and branched alkyl groups have between about 1 and 20 carbons; the substituted and unsubstituted cycloalkyl groups have between about 4 and 10 carbons and the substituted and unsubstituted aryl groups have between about 6 and 20 carbons. For example, where xe2x80x98Rxe2x80x99 is an alkyl group, xe2x80x98Rxe2x80x99 includes but is not limited to methyl, chloromethyl and ethyl groups, and the normal and branched propyl, 2-chloropropyl, butyl, pentyl and hexyl groups. Where xe2x80x98Rxe2x80x99 is a cycloalkyl group, xe2x80x98Rxe2x80x99 includes but is not limited to cyclopentyl, cyclohexyl, chlorocyclohexyl and cycloheptyl groups; where xe2x80x98Rxe2x80x99 is an aryl group, xe2x80x98Rxe2x80x99 includes but is not limited to phenyl, naphthyl, tolyl and benzyl groups. It will be understood, that the specific carbon content of any specific organohydridosiloxane resin, in accordance with this invention, is a function of the mole ratio of organotrihalosilane(s) to hydridotrihalosilane starting materials employed. Thus, for any xe2x80x98Rxe2x80x99 group selected, a resin in accordance with the present invention having a carbon containing substituent present in an amount of at least 40 Mol % is provided.
Advantageously, embodiments in accordance with the present invention are polymers having a caged structure with a polymer backbone encompassing alternate silicon and oxygen atoms. In particular, each backbone silicon atom is bonded to at least three backbone oxygen atoms to form the aforementioned cage structure. Essentially all additional silicon bonds are only to hydrogen and the organic substituents defined in Formulae 1, 2, 3 and 4. Thus, polymers of the present invention have essentially no hydroxyl or alkoxy groups bonded to backbone silicon atoms and cross-linking reactions are suppressed.
In contrast, previously known organosiloxane resins have high levels of alkoxy groups bonded to backbone silicon atoms, thus significant hydrolysis to form silanol groups is observed. This hydrolysis results in higher dielectric constants for the as-cured polymer films formed from these previously known resins, as well as reduced shelf life of solutions of these resins. The latter due to unwanted chain lengthening and cross-linking.
Thus embodiments of the present invention, by providing only hydrogen and organic groups directly bonded to backbone silicon atoms, avoid unwanted chain lengthening and cross-linking caused by condensation of the hydroxyl or silanol groups. Consequently, the shelf life of solutions of organohydridosiloxane resins in accordance with this invention is significantly prolonged over similar solutions of the previously known resins.
In accordance with the methods of this invention, the synthesis of the organohydridosiloxane compositions of this invention include a dual phase solvent system using a catalyst. In some embodiments of the present invention, the starting materials encompass trichlorosilane and one or more organotrichlorosilanes, for example organotrichlorosilanes having the substituted and unsubstituted groups defined with respect to Formulae 1 to 4, above.
In some embodiments, the catalyst is a phase transfer catalyst including, but not limited to, tetrabutylammonium chloride and benzyltrimethylammonium chloride. For example, bromide, iodide, fluoride or hydroxide anions are employed in some embodiments in place of the previously mentioned chloride anions. The phase transfer catalyst is introduced into the reaction mixture and the reaction is allowed to proceed to the desired degree of polymerization.
In other embodiments the catalyst is a solid phase catalyst, such as Amberjet 4200 or Amberlite I-6766 ion exchange resin (Rohm and Haas Company, Philadelphia, Pa.). Amberjet 4200 and Amberlite I-6766 are basic anion exchange resins. By way of explanation, and not by way of limitation, it is believed that the resin facilitates the hydrolysis of the Sixe2x80x94Cl bonds of the monomer to Sixe2x80x94OH. The hydrolysis is followed by condensation of two Sixe2x80x94OH moieties to provide an Sixe2x80x94Oxe2x80x94Si bond.
In accordance with one aspect of the method of this invention, a dual phase solvent system includes a continuous phase non-polar solvent and a polar solvent. The non-polar solvent includes, but is not limited to, any suitable aliphatic or aromatic compounds or a mixture of any or all such suitable compounds, the operational definition of xe2x80x9csuitablexe2x80x9d in the present context includes the functional characteristics of:
1) solubilizing the monomeric trihalosilane compounds,
2) solubilizing the organohydridosiloxane resin products as they are formed and increase in molecular weight,
3) stability of the organohydridosiloxane resin products in the solvent, and
4) insolubility of unwanted reaction products in the non-polar solvent.
Exemplary non-polar solvents include, but are not limited to, pentane, hexane, heptane, cyclohexane, benzene, toluene, xylene, halogenated solvents such as carbon tetrachloride and mixtures thereof.
The polar phase, is immiscible with the non-polar solvent phase, and includes water, alcohols, and alcohol and water mixtures. The amount of alcohol present is sufficient to ensure sufficient solubility of the organotrihalosilane monomers.
It has been found that a polar solvent to non-polar solvent ratio of between about 5 percent weight to weight (% w/w) to 80% w/w is desirable and between about 9% w/w to about 40% w/w preferred.
Exemplary alcohols and other polar solvents suitable for use in the polar phase include, but are not limited to, water, methanol, ethanol, isopropanol, glycerol, diethyl ether, tetrahydrofuran, diglyme and mixtures thereof. In one embodiment, the polar solvent includes a water/alcohol mixture wherein the water is present in an amount sufficient to preferentially solubilize ionic impurities not soluble in alcohol, and/or preclude solvent extraction of product compounds that might otherwise be soluble in alcohol. The polar solvent phase advantageously retains the hydrochloric acid (HCl) condensation product and any metal salt or other ionic contaminants, that may be present. As essentially all ionic contaminants are retained in the polar solvent phase, the organohydridosiloxane product of this invention is of high purity and contains essentially no ionic contaminants.
It will be understood, that in addition to retaining HCl condensation products and other ionic contaminants in the polar phase, the methods of the present invention also provide for high purity organohydridosiloxane product by avoiding sources of ionic contamination. Thus, in contrast to the methods for making the previously known organosiloxane resins, methods in accordance with the present invention do not employ metal catalysts or very strong inorganic acids, e.g. fuming sulfuric acid. In this manner, the extraction or leaching of metal contaminants by such strong acids or inclusion of metal catalyst residues are avoided and high purity organohydridosiloxane product obtained.
A mixture of the organic and hydridosilanes (e.g. trichlorosilane and methyltrichlorosilane) is added to a mixture of catalyst, hydrocarbon solvent, alcohol and water. The mixture is filtered, the water is separated, the solution is dried and then evaporated to leave a white solid. This solid is slurried in hydrocarbon solvent to remove monomer and then evaporated to leave desired product that can be formulated in a suitable solvent for use as a spin-on polymer. The molecular weight (Mw) of the product produced can be varied between 400 and 200,000 atomic mass units (amu) depending on the reaction conditions. It has been found that materials with a Mw of between approximately 5,000 to 60,000 amu are desirable. It has also been found that materials with a Mw of between approximately 10,000 to 50,000 amu are somewhat more desirable and materials with a Mw of between approximately 20,000 to 40,000 amu are most desirable.
The following characteristics encompass non-limiting measurements that illustrate the properties of organohydridosiloxane polymer resins of the present invention. The methods of measurement used are as follows:
1) Film Thickness (A): Film thickness is measured using a calibrated Nanospec(copyright) AFT-Y CTS-102 model 010-180 Film Thickness Measurement System available from Nanometrics, Co. An average of measurements at five locations on a wafer are reported as the film thickness for each sample.
2) Molecular Weight (xe2x80x9cMWxe2x80x9d): Molecular weight is determined using a gel phase chromatography system from Waters Corporation, Milford, Mass., equipped with a Waters 510 pump, Waters 410 differential refractometer and a Waters 717 autosampler. As is the customary practice in the field of Silicon polymers, weight average molecular weight is reported. The procedure used is as set forth by S. Rosen in xe2x80x9cFundamental Principles of Polymeric Materials,xe2x80x9d pages 53-81, (2nd Ed. 1993) and incorporated herein by reference.
3) Dielectric Constant: Dielectric constant is determined using the capacitance-voltage (xe2x80x9cCVxe2x80x9d) measurement technique and employs a Hewlett-Packard Model 4061A semiconductor measurement system at a frequency of 1 MHz. This test procedure employs a metal-insulator-metal (MIM) structure with the thickness of each layer ranging from about 0.5 to 1 micron (xcexcm).
A mixture of the organic and hydridosilanes (e.g. trichlorosilane and methyltrichlorosilane) is added to a mixture of catalyst, non-polar solvent, and polar solvent to form a reaction mixture. The polymerization reaction is allowed to proceed. Upon completion of the polymerization reaction, the reaction mixture is filtered, the polar solvent is separated, and the solution is dried and then evaporated to leave a white solid. This solid may then be slurried in a hydrocarbon solvent to remove residual monomer, and finally evaporated to leave the desired product. In some embodiments of the present invention, organohydridosiloxanes are formulated in a suitable solvent for use as a spin-on-dielectric film.